The present invention relates to devices for measuring by means of elastic waves the acceleration of a moving body, the accelerometers being connected to this moving body.
Measuring the acceleration by the inertial force exerted by a mass called seismic mass is known.
Measuring forces by the frequency variation of elastic wave oscillators subjected to this force is also known. Devices for measuring the forces applied to a flexed piezoelectric wafer having acoustic surface wave delay lines on each of its faces are described in U.S. Pat. No. 3,878,477 filed on Jan. 8, 1974 in the name of Hewlett-Packard.
The force applied in these devices is perpendicular to the wafer and it is measured by the frequency difference between the two oscillators comprising these acoustic lines.
The performance of an accelerometer comprising a seismic mass exerting a force on a wafer having delay line oscillators on both its faces is defined, on the one hand, by the sensitivity expressed as a frequency variation per unit of acceleration (generally 9.81 m/sec.sup.2 is taken) and, on the other hand, by the passband B of the accelerometer, when the acceleration varies as a function of time.
The disadvantage of an accelerometer comprising a wafer subjected to flexing is that it only allows passbands of a few hundred Hz, if not the sensitivity becomes too low and this despite the spectral purity of the surface wave oscillators.
Another disadvantage is that the flexure stresses undergone by the wafer are not homogeneous either in the direction of propagation of the waves or in the thickness of the substrate.
The accelerometer of the invention does not have these disadvantages and, in particular, it enables good sensitivity to be obtained for passbands as high as 10 kHz.
The accelerometer of the invention comprises a wafer which, on account of the inertial forces, has one part compressed and the other extended. To each one of the two parts there corresponds a surface wave oscillator. The device of the invention has moreover the advantage of an homogeneous stress which improves the spectral purity of the oscillator.